1. Field of the Invention
The present invention relates to a cantilever type probe for a scanning tunnel microscope (hereinafter referred to as "STM"), and a scanning tunnel microscope provided with the probe.
The present invention further relates to an information-processing apparatus which records, reproduces and erases information by an STM technique employing the above cantilever type probe.
2. Related Background Art
In recent years, scanning tunnel microscopy has been developed which enables direct Observation of the electronic structure of atoms on a surface of a conductor (G. Binnig, et al.: Phys. Rev. Lett. 49, (1982) 57). With this technique, real spatial images of materials, whether single crystalline or amorphous, can be measured with extremely high resolution (nanometer or less). The STM utilizes a tunnel current which flows between a metallic tip and an electroconductive substance when a voltage is applied between the probe and the substance which are brought as close as about 1 nm. This tunnel current is highly sensitive to the change of distance between the two, and varies exponentially. Accordingly, real spatial surface structure can be observed with a resolution of atomic order by scanning the surface with the probe while keeping the tunnel current constant. The analysis by use of the STM is limited to electroconductive materials. However, the STM is coming into use for structure analysis of a thin insulation film formed thin on a surface of an electroconducitve materials. Such an apparatus and method are advantageous in that the observation can be conducted with low electric power without impairing the medium since the apparatus and the method utilize detection of extremely weak current. Furthermore, the apparatus can be operated in the atmosphere, and are promising for a variety of application fields.
In particular, the technique is becoming practical for high density recording-reproducing, as disclosed in Japanese Patent Application Laid-Open Nos. 63-161552, 63-161553, etc. This method employs a probe electrode similar to that of STM, and conducts recording by varying the voltage between a probe and a recording medium, the recording medium being a material exhibiting memorizable switching characteristics in its voltage-current characteristics, e.g., thin films of chalcogens and .pi.-electron type organic compounds. In this method, the reproduction is conducted by utilizing the difference of the tunnel resistance between recorded areas and non-recorded areas. In such a recording system, a recording medium that changes its surface shape with the voltage applied to the tip may be used.
In the operation of the STM or the recording and reproducing by application of the STM, it is important to control the distance between the specimen and the sample or the recording medium in the order of angstroms, and to control the two-dimensional scanning with an accuracy of several ten angstroms in recording and reproducing the two-dimensionally arranged information. Furthermore, simultaneous driving of a plurality of probes (namely, multiplication of the probe) is proposed to improve the functionality, particularly, to speed-up the recording-reproduction system. With the multiple probes, the relative position of the probe and the medium has to be three-dimensionally controlled with the accuracy mentioned above within the area where the multiple probes are used.
Heretofore, the control of the probe has been conducted by using a lamination type piezoelectric element or a cylindrical piezoelectric element attached at the probe side or the medium side. Such elements, which are capable of larger displacement, are not suitable for integration, and are disadvantageous in use for the multiprobe type recording-reproducing apparatus. The tip module may be fitted onto a cantilever of several hundred .mu.m long, and the cantilever may be driven by a piezoelectric element to avoid this disadvantage.
In a known method, a cantilever having a multilayer structure constituted of a thin piezoelectric film, a metallic film, and so forth is prepared by utilizing a technique of forming a fine structure on a substrate in a semiconductor production process (T. R. Albrecht, et al.: "Microfabrication of Integrated Scanning Tunneling microscope", Proceedings Fourth International Conference on Scanning Tunneling Microscopy/Spectroscopy, 1990).
FIG. 4 is a perspective view of a conventional cantilever type probe. The probe includes piezoelectric thin films 101, 102; piezoelectric film-driving electrodes 103, 103', 105, 106, 106': a tip 108; a probe leader electrode 109; a silicon substrate 100; and a thin Si.sub.3 N.sub.4 film 113. FIG. 3 is a block diagram of an STM apparatus employing the cantilever type probe. The STM of the block diagram includes a bias applying power source 201, a tunnel current amplification circuit 202, a cantilever driver 203, a cantilever 204, a probe 205, and a test specimen 206. The tunnel current I.sub.t flowing between the probe 205 and the specimen 206 is detected, and the cantilever is driven by application of feedback so as to keep I.sub.t constant, whereby the gap between the probe 205 and the specimen 206 is kept constant.
In conventional cantilever type probes piezoelectric film-driving electrodes including an upper electrode, a lower electrode, and an intermediate electrode formed of the same width. Consequently, positional deviation 110 of the driving electrodes, as shown in FIG. 5, in forming the pattern of the electrodes is caused by an aligning error, which disadvantageously leads to a change of the effective electrode areas. This positional deviation arises in a mask alignment in photolithography process, and is usually within .+-.3 .mu.m depending on the mask aligner employed. This positional deviation makes the area between the counter electrodes in the displacement regions 111, 111' smaller than that in the displacement regions 112, 112', which reduces the displacement, and disadvantageously result in twisting of the cantilever. Therefore, when this cantilever is used in STM operation or STM recording-reproducing apparatus, the moving direction of the cantilever is changed to cause positional deviation relative to the coordinate of access, making the stable operation thereof difficult. When the twisting is larger, the end portion of the cantilever may be brought into contact with the specimen rather than the probe, which may damage the specimen or the recording medium.